Loans and allowance for credit losses - Loans Outstanding by Major Lending Classification (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Gross loans	$ 7,213,538		$ 4,409,642	$ 4,409,642
Less: Allowance for credit losses	(183,973)	$ (113,129)	$ (31,139)	(31,139)	$ (31,464)	$ (30,138)	$ (28,932)
Net loans	7,029,565			4,378,503
Loans processed	310,719
Paycheck Protection Program
Financing Receivable, Past Due [Line Items]
Less: Allowance for credit losses	(49)
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Gross loans	1,417,671			1,034,036
Less: Allowance for credit losses	(8,609)	(8,878)		(4,805)	(5,170)	(4,923)	(5,348)
Construction
Financing Receivable, Past Due [Line Items]
Gross loans	1,190,878			551,101
Less: Allowance for credit losses	(63,629)	(35,599)		(10,194)	(9,842)	(9,655)	(9,729)
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	1,140,611			710,454
Less: Allowance for credit losses	(18,298)	(12,463)		(3,112)	(3,331)	(3,288)	(3,428)
Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Gross loans	420,318			221,530
Less: Allowance for credit losses	(11,455)	(6,811)		(752)	(727)	(755)	(811)
Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	165,937			69,429
Less: Allowance for credit losses	(7,191)	(4,499)		(544)	(574)	(617)	(566)
Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	924,987			630,270
Less: Allowance for credit losses	(6,705)	(7,420)		(4,109)	(3,709)	(3,512)	(3,132)
Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	1,644,400			920,744
Less: Allowance for credit losses	(57,640)	(30,444)		(4,621)	(4,927)	(4,478)	(4,149)
Consumer and other
Financing Receivable, Past Due [Line Items]
Gross loans	308,736			272,078
Less: Allowance for credit losses	$ (10,446)	$ (7,015)		$ (3,002)	$ (3,184)	$ (2,910)	$ (1,769)